Solid-state organic light emitting diode (OLED) image displays are of great interest as a superior flat-panel display technology. These displays utilize current passing through thin films of organic material to generate light. The color of light emitted and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin-film material. Different organic materials emit different colors of light. However, as the display is used, the organic materials in the device age and become less efficient at emitting light. This aging effect is dependent on the temperature of the OLED display and is increased at higher temperatures. It is useful, therefore, to provide a means to reduce the temperature of an OLED device in operation.
OLED devices are conventionally either top-emitting or bottom-emitting. An OLED display is constructed upon a substrate by depositing a first electrode upon the substrate, organic light emitting materials over the first electrode, and a second electrode above the light emitting materials. A cover is used to encapsulate and protect the device. Light is emitted by the application of a current from one electrode to another passing through the organic light emitting materials. A bottom-emitting device emits light through the substrate and first electrode, which must both be transparent. The second electrode may be either transparent or reflective. A top-emitting device emits light through the cover and second electrode, which must both be transparent. In this case, the first electrode may be either transparent or reflective.
A variety of materials may be used to construct suitable substrates and encapsulating covers for OLED devices and to fill the cavity between the second electrode and the cover. The desirable material properties and/or characteristics of an OLED substrate and cover include low cost, very flat surface, low coefficient of thermal expansion (CTE), high strength and stability under a variety of environmental stresses, and electrically non-conductive (or able to be coated with electrically non-conductive material). The material used most often for such substrates is glass, typically borosilicate glass, because it is transparent, very stable, can be made at low cost, and has a very smooth surface suitable for the deposition and processing of semiconductor and organic materials. Other substrate materials, both rigid and flexible, have been described in the art, for example ceramics, plastics, and metals such as stainless steel (see U.S. Pat. No. 6,641,933 B1 to Yamazaki et al. entitled “Light-Emitting EL Display Device”).
Organic light-emitting diodes can generate efficient, high-brightness displays. In operation, OLED devices are provided with a voltage differential across the electrodes by an external power supply either directly (as in a passive-matrix OLED device) or through a locally controlled power circuit (as is found in an active-matrix device). The voltage differential causes a current to flow through the OLED materials, which then causes the OLED materials to emit light. However, the conversion of current to light is relatively inefficient, so that much of the energy is converted to heat. Moreover, much of the emitted light does not escape from the OLED device and is reabsorbed into the device as heat. Hence, OLED devices can become very hot and operate at temperatures well in excess of ambient temperatures. For example, in an ambient environment of 20° C., applicants have demonstrated that an OLED may operate at 40° C. to 60° C. This heat is detrimental to the OLED device. As is well known, OLED materials degrade as they are used and degrade faster at higher temperatures. Therefore, providing improved heat management to cool an OLED device improves the lifetime of the OLED device.
While it is important to maintain the overall brightness of an OLED display, it is even more important to avoid localized degradation within a display. The human visual system is acutely sensitive to differences in brightness in a display. Hence, differences in uniformity are readily noticed by a user. Such localized differences in uniformity in an OLED display may occur as a consequence of displaying static patterns on the display, for example, graphic user interfaces often display bright icons in a static location. Such local patterns will not only cause local aging in an OLED display, but will also create local hot spots in the display, further degrading the light-emitting elements in the local pattern. Glass and plastic supports, the use of which is advantageous in view of their relative electrical non-conductivity, may not be sufficiently thermally conductive to provide a uniform temperature across the substrate when the display is in operation. Hence, improved thermal management techniques may significantly improve the life expectancy of an organic display device.
One method of removing heat from an organic light emitting display device is described in U.S. Pat. No. 6,265,820 to Ghosh et al., entitled, “Heat Removal System for Use in Organic Light Emitting Diode Displays Having High Brightness.” The '820 patent describes a heat removal system for use in organic light emitting diode displays. The heat removal assembly includes a heat dissipating assembly for dissipating heat from the organic light emitting device, a heat transfer assembly for transferring heat from the top organic light emitting device to the heat dissipating assembly and a cooling assembly for cooling the organic light emitting display device. While the system of the '820 patent provides a means for heat removal in an OLED application, its efficiency is limited by the presence of a glass substrate having poor thermal conductivity characteristics through which heat generated by the OLED devices must transfer for removal. Moreover, the structure described in the '820 patent is complex, requiring multiple layers and specific heat transfer materials in contact with delicate OLED layers.
U.S. Pat. No. 6,480,389 to Shie et al. entitled “Heat Dissipation Structure for Solid-State Light Emitting Device Package” describes a heat dissipation structure for cooling inorganic LEDs characterized by having a heat dissipating fluidic coolant filled in a hermetically sealed housing where at least one LED chip is mounted on a metallic substrate within a metallic wall erected from the metallic substrate. U.S. Pat. No. 5,821,692 to Rogers et al. entitled “Organic Electroluminescent Device Hermetic Encapsulation Package” describes an organic electroluminescent device with a dielectric liquid filling the space between the cover and the organic electroluminescent device. Such arrangements are complex, require fluids, and are difficult to construct in devices such as OLEDs.
U.S. Patent Application Publication 2004/0004436 A1 to Yoneda entitled “Electroluminescent Display Device”, describes an organic electroluminescent (EL) panel having a device glass substrate provided with an organic EL element on a surface thereof, a sealing glass substrate attached to the device glass substrate, a desiccant layer formed on a surface of the sealing glass substrate, and spacers disposed between a cathode of the organic EL element and a desiccant layer. A heat-conductive layer can be formed by vapor-depositing or sputtering a metal layer such as a Cr layer or an Al layer that inhibits damaging the organic EL element and increases a heat dissipating ability, thereby inhibiting aging caused by heat. U.S. Pat. No. 6,633,123 to Tazawa entitled “Organic Electroluminescence Device with an Improved Heat Radiation Structure.” describes an organic electroluminescence device having a heat radiation layer higher in heat conductivity than the substrate. However, such structures, while useful, do not assist in removing heat from the electroluminescent element itself.
Heat sinks are also well known in the integrated circuit industry and are applied to cooling large integrated circuits. Such sinks typically are thick and are unsuitable for displays in which limiting the thickness of the display is an important goal.
It is therefore an object of the present invention to provide a more uniform distribution of heat within an OLED display and to improve the removal of heat from an OLED display device thereby increasing the lifetime of the display, while maintaining the simplicity of the design and manufacturing process as well as the thinness of the display.